[boost] Re: [any] checked convertion to reference?

From: Vladimir Prus (ghost_at_[hidden])
Date: 2005-02-24 05:20:48

Alexander Nasonov wrote:

>
> Vladimir Prus wrote:
> > Alexander Nasonov wrote:
> > I think you're right, and trying gcc +
> > BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION confirms that.
> > I've committed this part of the patch.
>
> Great!
>
> One question. How to make sure that we don't break existing code on broken
> compilers? 

To clarify: the change I've committed only adds cast to reference. I
postponed 
the other parts until we finish our discussion. So for now, all old code 
still works. 

> My plan is to extend existing testcase with more exotic use of 
> _old_ functionality (details below) and to put _new_ functionality into
> separate testcase. Does it sound good to you?

The only new functionality that won't work on broken compilers is cast to 
reference of user-defined type for which remove_reference was not specified. 
Do you think we need to test for this explicitly?

> > > const is already stripped because ctor's argument is 'const ValueType
&
> > > value'.
> > > So, it's only for volatile. remove_cv is used only to avoid inclusion
> > > of remove_volatile.hpp.
> >
> > Does this use case matters, btw?
>
> I don't know. Even if it matters to some, I never saw their complains.

Neither did I.

> Presumably, because this code snipset compiles and runs nicely:
>
> š š // candidate for inclusion into _old_ test
> š š int volatile i = 0;
> š š any a_i(i);
> š š any_cast<int>(a_i);
> š š int r = any_cast<int>(a_i);
> š š r = any_cast<int const>(a_i);
> š š r = any_cast<int volatile>(a_i);
> š š r = any_cast<int const volatile>(a_i);
>
> If not patched, it has a suble bug. a_i owns holder<int volatile> but
> any_cast function treats it as holder<int>, holder<int const>, holder<int
> volatile> and holder<int const volatile>, respectively. 

Yea. I think it will never bite in practice, though, because 'const' or 
'volatile' won't affect the object layout and IIRC, 'typeid' strips
top-level 
cv qualifiers.

> If you're going to 
> fix this, please do it on both ends: in ctor and in any_cast. If you fix
> only any_cast end, you may end up with unsafe down cast over base pointer
> from holder<int volatile> to holder<int>. If you disable volatile at ctor
> end, some users (better if tests) will complain.
>
> Correct type for storing 'int cv' value in a_i is holder<int> (without
> cv!).

Yes, that's true.

> > What about declaring that 'T' in all variants should have *no*
> > cv-qualification? It's a bit strange that the user can cast to volatile
> > type, and the any_cast will just throw that away. And cast *from*
> > volalite any does not work, i.e.
> >
> > š š volatile boost::any a = 1;
> > š š volatile int& i = boost::any_cast<volatile int&>(a);
> >
> > does not compile. So what's the point of allowing volatile type at all?
>
> Why do you need volatile any? Safe casts to cv types and proper
> construction from volatile variable are things I was trying to fix.

I don't need volatile any, that's the point. I find it clearer if docs say 
'You can pass cv-qualified types to 'any_cast'', then if they say 'you can 
cast to volatile type but you can't cast from volatile any'.

- Volodya